The find-large-objects scrubber subcommand is quite fast if you run it
in an environment with low latency to the S3 bucket (say an EC2 instance
in the same region). However, the higher the latency gets, the slower
the command becomes. Therefore, add a concurrency param and make it
parallelized. This doesn't change that general relationship, but at
least lets us do multiple requests in parallel and therefore hopefully
faster.
Running with concurrency of 64 (default):
```
2024-07-05T17:30:22.882959Z INFO lazy_load_identity [...]
[...]
2024-07-05T17:30:28.289853Z INFO Scanned 500 shards. [...]
```
With concurrency of 1, simulating state before this PR:
```
2024-07-05T17:31:43.375153Z INFO lazy_load_identity [...]
[...]
2024-07-05T17:33:51.987092Z INFO Scanned 500 shards. [...]
```
In other words, to list 500 shards, speed is increased from 2:08 minutes
to 6 seconds.
Follow-up of #8257, part of #5431
Adds a find-large-objects subcommand to the scrubber to allow listing
layer objects larger than a specific size.
To be used like:
```
AWS_PROFILE=dev REGION=us-east-2 BUCKET=neon-dev-storage-us-east-2 cargo run -p storage_scrubber -- find-large-objects --min-size 250000000 --ignore-deltas
```
Part of #5431
The S3 scrubber contains "S3" in its name, but we want to make it
generic in terms of which storage is used (#7547). Therefore, rename it
to "storage scrubber", following the naming scheme of already existing
components "storage broker" and "storage controller".
Part of #7547
